The invention has evolved from research on precision range measurement by the use of a heterodyne technique of phase sensing of radio frequency modulated optical laser signals, the subject of U.S. Pat. No. 5,162,862 dated Nov. 10, 1992.
The measurement of fluctuations in the intensity of an electrical signal, particularly an optical signal, with the purpose of sensing the degree of attenuation owing to the position of some obstructing object in the signal path, is extremely difficult if the measurement is to be divorced from fluctuations of the signal source intensity. Power supply variations and temperature variations at the signal source are a major problem confronting the design of occultation systems in the machine tool industry. Whenever active components are used in electronic circuits, as for introducing a delay, the power supplies and inherent stability problems of the components can seriously impair performance.
The invention to be described exploits a technique designed to measure the phase difference between two signals, but applies this instead to a measure of relative attenuation of those signals. The resulting structure is novel and the tested result is a surprising and unexpectedly precise measure of signal attenuation with very high resolution. The technique makes possible measurements which have not hitherto been possible in industrial applications. By avoiding the use of circuits and special components previously deemed essential in such applications, there are very substantial cost savings and so the invention, in one of its applications, provides a major step forward in machine tool technology.
It is well known to measure phase by amplitude measurement and addition and this is the most common method of determining the phase difference between two equal amplitude sinusoidally varying signals. This technique is reasonably cheap to implement but has limited resolution since amplitude measurement is notoriously difficult to isolate from external influences. For this reason phase measurement is better accomplished by direct timing of the interval between the zero crossing point of one sine wave signal and the zero crossing point of the other. The resolution is then limited by the ratio between the much higher frequency of a timing clock and the frequency of the measured waves.
There is in this latter case also the problem of the speed and accuracy with which the zero crossing points can be sensed, but a very substantial advance in the technology of such measurement has recently been achieved and is the subject of the above-referenced U.S. Pat. No. 5,162,862.
The subject invention develops the latter technology in a novel way by using it indirectly as a phase measure which can, by a reverse mode of measurement, provide a measure of signal intensity attenuation cheaply and with high precision and high resolution.
Usually, when the signals are at high frequency then the phase or intensity attenuation measurement problem becomes far more difficult. However, as will be seen, a high frequency system offers scope for the use of passive components in a special way in the implementation of this invention. The result is an overwhelming advantage for the high frequency system. Indeed, the high frequency has proved a key contributer to the quality of resolution and the reliability and precision of the measurement, whilst the technology of U.S. Pat. No. 5,162,862 is brought to bear to assure precision in the phase quantity which translates into a signal attenuation measure.
As already stated, a principal specific application of the invention is in connection with photoelastic load cells but the description below will first be concerned with the more general application to sensors in precision machine tools. Here the measurement of linear displacements is now routine in automated processes. An automatic lathe, for example, may involve what are known as Moire gratings in which one grating is attached to the work and moves across a stationary grating. Light that has passed through the gratings is received by a photo-cell and varies in intensity as the movement of the work causes a Moire fringe to cross the field sensed by the photo-cell. The output of the photo-cell controls the displacement of the work in a predetermined manner as part of the regulated machining process, but the operation relies on the precise measure of position interpolated to fractions of a fringe distance. That measure is judged in terms of the light intensity as a proportion of the maximum intensity. Hence the need for precision measurement of electric signal intensity control systems used in the machine tool industry.
Another typical application is concerned with the measure of change of resistance in strain gauges attached to pipework in chemical plants, oil rigs etc. Here the variation of electric signal strength relative to a datum signal becomes a measure of mechanical strain which affects that resistance. However, the strain gauge need not be a resistance strain gauge, but could, within the terms of this invention, become one based upon optical techniques for measuring displacement, possibly also using the Moire fringe gratings. In either case, however, there is the need to measure the degree to which the intensity of a signal is affected by an attenuating action that is a measure of the physical displacement in which one is interested.
The subject of the above-reference U.S. Pat. No. 5,162,862 is a heterodyne conversion technique having novel phase-locked loop connections coordinating a master transmitter oscillator and two receiver circuits under the control of another oscillator. The disclosure suggested application as an optical radar measurement system by which the flight time of light reflected from a target is used as the measure of range. There are features of that prior invention that find application in a structure embodying the subject invention and which are essential to realise its primary advantages but, as will be seen, further inventive concepts are needed to provide an operative system as defined by this invention.